Jus picked up a set of speaker adapters for the front of the astra, read the how to guides and it seems like a right job, taking the door cards off to get to the speaker!
In my 106, i had 4 torx screws to undo and the door bin came off, and there was the speaker, easy peasy!
Im wondering if its even worth the hassle now!
Keep a look out, i may have some speaker adapters up for sale soon! haha!